1. Field of the Invention
In at least one aspect, the present invention relates to NOx traps with reduced NOx release during rich purges, increased NOx conversion efficiency under stoichiometric conditions, and improved sulfur tolerance and desulfation capability.
2. Background Art
Current three-way catalysts are effective for converting the HC, CO, and NOx in the exhaust into CO2, H2O, and N2 when the air fuel (“A/F”) ratio is controlled about the stoichiometric mixture of 14.6 to 1. With the closed-loop control systems used in modern vehicles, the A/F ratio actually alternates between a slightly lean condition and a slightly rich condition with a frequency of about 1-2 Hz and at an amplitude of approximately 0.3 to 0.5 A/F ratio units. To provide high three-way conversion in this oscillatory environment, three-way catalysts usually contain oxides of cerium or mixed oxides of cerium and zirconium. For the short periods of slightly lean exhaust, the cerium is able to adsorb the excess oxygen, allowing the NOx reduction to continue. The cerium also releases oxygen during the short periods of slightly rich exhaust, providing oxidants for converting the HC and CO. The combination of tight A/F ratio control near stoichiometry and the modern three-way catalyst provide very high conversion efficiencies of the HC, CO, and NOx and allow automakers to satisfy stringent emission legislation in markets around the world.
It is desirable to operate the engine lean in order to improve the fuel economy. By opening up the air throttle plate and operating the engine with excess air, the pumping losses across the throttle plate are reduced and the thermodynamic efficiency of the engine is improved, resulting in decreased fuel consumption. However, the exhaust from such an engine contains large amounts of excess oxygen for extended periods of time (e.g., 30-60 seconds), and current three-way catalysts are unable to provide the NOx control necessary to satisfy stringent emission legislation in this environment.
One potential solution to this emission and fuel economy dilemma is to use a lean NOx trap. Lean NOx traps are three-way catalysts and, like all such catalysts, can store NOx under lean conditions for limited periods of time. NOx traps contain alkaline earth or alkali metals to enhance their NOx storage capabilities under lean conditions. Such catalysts can store NOx with high efficiency for a period of time on the order of 60 seconds. Periodically, as the NOx capacity of the NOx trap is approached, the A/F ratio must be driven to a rich condition for a few seconds in order to purge and reduce the stored NOx and regenerate the NOx storage capacity of the trap.
One of the characteristic features of lean NOx traps is that they are most effective at storing NOx in a temperature window that can vary somewhat with the formulation but is typically between 200° C. and 550° C. As a result of this temperature sensitivity, the NOx traps are typically placed in the underfloor location in the exhaust. Lightoff catalysts can be placed close to the exhaust manifold to provide fast lightoff during a cold-start. These lightoff catalysts can be formulated with little or no oxygen storage capacity (“OSC”) in order to minimize the fuel required to purge and regenerate the lean NOx trap.
Lean NOx traps can provide very high conversion of NOx when the engine is operated with an A/F ratio control strategy consisting of extended periods of lean operation with periodic rich purges. However, the catalyst system is also expected to provide high three-way conversion when the A/F ratio is controlled at stoichiometry, for example during high load operation. If the close-coupled catalysts contain low amounts of OSC, this limits the ability of these catalysts to convert CO and NOx under stoichiometric conditions. Therefore, unless there is a cerium-containing three-way catalyst downstream of the NOx trap, the NOx trap itself must contain some OSC in order to provide high CO and NOx conversion under the oscillatory A/F conditions characteristic of closed-loop control systems.
The presence of cerium in the Nox trap has been observed to provide other benefits besides improving the stoichiometric performance of the trap. The cerium can improve the sulfur tolerance of the trap by adsorbing some of the sulfur and preventing that portion of the sulfur from poisoning the NOx storage sites. The cerium also improves the desulfation characteristics of the trap by promoting the water-gas-shift (WGS) reaction. The WGS reaction produces additional hydrogen, which has been shown to be the best agent for desulfating the poisoned trap. In addition, the presence of cerium can improve the NOx storage capability at low temperatures, as cerium is able to provide some NOx storage capacity at low temperatures (e.g., 300° C.). Finally, the cerium can be beneficial for the thermal durability of the trap, as ceria is known to stabilize the dispersion of the precious metals.
However, the presence of cerium in the trap can also be responsible for some undesirable effects. As with the lightoff catalysts, cerium in the trap requires additional reductants (i.e., HC, CO, H2) to purge the NOx trap, increasing the fuel penalty associated with the purges. A second undesirable effect, which is the subject of this invention, is that the oxygen storage capacity provided by the cerium can cause some of the stored NOx to be released from the trap during the purges without being reduced to N2. This purge NOx release is particularly evident at temperatures of 350° C. and above. A major source of this NOx release is attributed to the exotherm that results from the reaction between the reductants in the exhaust and oxygen from the cerium during the transition from lean operation to the rich purge condition.
Accordingly, there exists a need for a lean NOx trap with a balanced amount of oxygen storage capacity that results in low levels of purge NOx release but still provides high NOx conversion under stoichiometric conditions and resistance to sulfur poisoning.